A sputter ion pump is a device for producing very high vacuum conditions. A conventional sputter ion pump comprises a vacuum envelope housing, at least one cathode electrode, an anode electrode formed as a plurality of hollow cylindrical cells, and means for applying to the anode a potential higher than that of the cathode. Sputter ion pump can be prvided with means for generating a magnetic field through the anode parallel to the axis of the cells.
In operation, when a potential is applied to the anode that is higher than the potential applied to the cathode, a region of intense electric field is produced between the cellular anode and the cathode that causes a breakdown of gas within the pump resulting in a glow discharge within the cellular anode, and between the anode and the cathode. This glow discharge results in positive ions being driven into the cathode electrode to produce dislodgment of reactive cathode material which is sputtered onto the nearby anode to produce gettering molecules in the gaseous stage coming in contact therewith. In this manner, the pressure within the vacuum envelope and, therefore, any container communicating therewith are evacuated.
To achieve an optimum operation of an ion pump operating at low pressures (p&lt;10.sup.-7 Torr) the anode cell radius R should be on the order of: ##EQU1##
where U is the voltage in Volts applied between the cathode or cathodes and the anode of the pump, B is the strength of the magnetic field inside the pump in Gauss, .nu..sub.i /.nu..sub.c is the ionization probability of an electron in a collision with a gas molecule (.nu..sub.i /.nu..sub.c.about.0.1 at pressures lower than 10.sup.-7 Torr) [Vacuum Science and Technology, Vol.11, No.6, November/December 1974].
Thus for an applied voltage of 5000 Volts and a magnetic field of 1150 Gauss, the radius R should be on the order of 1.07 cm.
Conventional anode cell structures are disclosed for example in the U.S. Pat. No. 4,631,002 issued to Pierini, and consist of a gathered cluster of cylindrical sectors. An array of cylindrical cells having radiuses equal or near equal to R leaves a number of inter-cylindrical cells having a generally triangular shape and a cross-section transverse dimension that is much smaller than R.
The typical diode sputter ion pumps display a class of instabilities that manifest as a mode shift phenomena following pump exposure to gas doses that are greater than the ultimate pressure of the vacuum system in which the pump is operating. Such mode shifting instabilities is disruptive to the devices to which the sputter ion pump is attached.
Irregular sputter-erosion patterns of the catode surface have been reported in diode sputter-ion pumps utilizing cylindrical cell anodes. Such irregular erosion are imputable to the inter-cylindrical cells and causes an increase of the pump dispersion current. The dispersion current effects are more evident when a pump has been used under high pressure conditions such as in electronic microscopes where the pump operation starts from high pressure levels.
Moreover, in general it is believed that mode instabilities may be caused by a loss of stability of the plasma in the oddly shaped inter-cylindrical cell of the anode structure. This arrangement might hinder a clean and quiet operation of the diode sputter ion pump.
A square anode cell pump that eliminates the intervening regions of a typical linked cylindrical cell design was suggested by Jepsen, as shown for example in the U.S. Pat. No. 3,319,875. Despite the advantage of having no intervening cell, the square cell anode design proved to be intrinsically inefficient. Moreover, the square cells have a larger area than that of a circle with radius R because of the presence of the peripheral comer areas.
It is an object of the present invention to provide a sputter ion pump provided with an anode electrode eliminating the above mentioned drawbacks of the prior art design.